Page 346 - Problem-Based Feline Medicine

338 PART 6 CAT WITH WEIGHT LOSS OR CHRONIC ILLNESS

it in capsules to administer immediately after a meal. ● The long-term advantages of this form of therapy
Calcium-containing phosphate binders such as cal- are as yet unproven. The aim is to decrease pro-
cium acetate (60–90 mg/kg/day divided with meals) gression of renal failure through a reduction in PTH
or calcium carbonate (90–150 mg/kg/day divided with concentrations. If there is marked hypercalcemia
meals) can be used, but serum calcium must be moni- based on measurement of ionized calcium, care
tored carefully. Do not use if there is hypercalcemia. should be taken to rule out other causes of hypercal-
● High dietary phosphate limits the effectiveness of cemia, because the hypercalcemia of renal failure is
phosphate-binding agents, so preferably these usually only mild to moderate in severity (total
should be used in combination with a low- calcium <3.4 mmol/L or 13.5 mg/dl).
phosphate diet.
Treatment of hypokalemia.
Treatment with an active form of vitamin D directly
Hypokalemia may be treated with oral potassium glu-
inhibits PTH secretion. This group of drugs has a
conate (2–6 mmol/cat/day PO). The exact dose is
very narrow therapeutic index.
dependent on the response to therapy, determined by
● Calcitriol or alphacalcidol 1.5–3.5 ng/kg/day PO
monitoring plasma potassium concentrations.
on an empty stomach (maximum 10 ng/kg/day).
Because of the very low dose, the human prepara- Treatment of the anemia.
tions must be re-formulated by a compounding
Management of the anemia of CRF should include an
pharmacy for use in cats.
initial assessment of the cat to rule out and treat other
● All active vitamin D preparations enhance intestinal
potential causes of anemia, such as flea infestation and
absorption of calcium and phosphorus, and so are
gastrointestinal ulceration.
contraindicated unless plasma concentrations of
calcium and phosphate are normal. In addition, they Nutritional and iron deficiencies will impair the ery-
are ineffective in decreasing PTH concentrations if thropoietic potential, and should be corrected prior to
phosphorus is elevated. Phosphorus concentration initiating other treatment.
must be below 2 mmol/L (6 mg/dl) before calcitriol
Blood transfusions (limited availability), androgen
use is optimal. Between serum phosphorus
(poor efficacy) and recombinant erythropoietin therapy
of 2–2.25 mmol/L (6–7 mg/dl), the effectiveness of
have all been used to treat the anemia of CRF.
calcitriol is decreased, and above a phosphorus
of 2.5 mmol/L (8 mg/dl) it is ineffective in reducing Recombinant human erythropoietin (rHuEPO) is
PTH concentration. the most effective therapy.
● Plasma calcium must be closely monitored, ini- ● Therapy should be started only if clinical signs
tially weekly. If possible measure ionized calcium relating to the anemia are present, such as weak-
rather than total calcium. The dosage should be ness, inactivity or lethargy, which is usually when
titrated to maintain normocalcemia, not merely to the packed cell volume (PCV) is less than 18%.
decrease PTH concentrations, and therapy should Because of potential side effects of treatment, it is
be discontinued if hypercalcemia occurs. If hyper- usually only commenced once PCV is <15%.
calcemia does not resolve within 7–10 days of ● Initial dosage is 50–100 U/kg injected subcuta-
stopping calcitriol, begin intermittent dosing of neously three times weekly, with PCV monitored at
calcitriol. For cats with mild hypercalcemia based least twice weekly. Once the PCV has increased to
on the ionized calcium concentration, give cal- the lower end of the reference range, the dosage is
citriol every other day at twice the daily dose, so usually reduced to once or twice weekly.
the total amount per week is the same. If calcium ● Adverse effects of rHuEPO include polycythemia,
levels do not decrease, try twice weekly dosing at seizures, hypersensitivity reactions and systemic
3.5 times the daily dose (8 pm one day and 8 am hypertension. Absolute failure to respond to
4 days later). Do not use less than twice weekly. rHuEPO is usually due to iron deficiency.
PTH concentrations should be measured to docu- ● After an initial response, many treated cats develop
ment successful control of renal secondary hyper- antibodies to rHuEPO, which leads to a severe
para-thyroidism. refractory anemia greater than that present prior to